There is a growing interest in depositing thin-film materials from gaseous precursors on a wide range of substrates for a wide variety of applications. Substrates of interest include both rigid substrates, such as flat-panel glass, and flexible substrates, such as plastic webs or metal foils. Flexible supports are of particular interest since they can be more mechanically robust, lighter weight, and allow for more economic manufacturing (e.g., by enabling roll-to-roll processing) than rigid substrates. Thin-film deposition systems, similar to their liquid coating counterparts, are advantaged if the deposition head, or gas delivery device, is smaller in area than the area of the substrate to be coated. For substrates that are continuous, such as webs and foils, the use of a deposition head that is smaller than the area of the substrate is a requirement not just an advantage.
Among the techniques widely used for thin-film deposition is chemical vapor deposition (CVD), which uses chemically reactive molecules that react to deposit a desired film on a substrate. Molecular precursors useful for CVD applications comprise elemental (atomic) constituents of the film to be deposited and typically also include additional elements. CVD precursors are volatile molecules that are delivered, in a gaseous phase, to a chamber in order to react at the substrate, forming the thin film thereon. The chemical reaction deposits a thin film with a desired film thickness.
Atomic layer deposition (ALD) is a thin-film deposition technology that provides excellent thickness control of conformal thin-films. The ALD process segments the thin-film deposition process of conventional CVD into single atomic-layer deposition steps. Advantageously, ALD steps are self-terminating and can deposit one atomic layer when conducted up to or beyond self-termination exposure times. An atomic layer typically ranges from about 0.1 to about 0.5 molecular monolayers, with typical dimensions on the order of no more than a few angstroms. In ALD, deposition of an atomic layer is the outcome of a chemical reaction between a reactive molecular precursor and the substrate. In each separate ALD reaction-deposition step, the net reaction deposits the desired atomic layer and substantially eliminates “extra” atoms originally included in the molecular precursor. In its most pure form, ALD involves the adsorption and reaction of each of the precursors in the absence of the other precursor or precursors of the reaction. In temporal vacuum ALD, thin-film growth is accomplished by alternating the delivery of two or more reactive materials, or precursors, into a vacuum chamber in time. Sequentially, a first precursor is applied to react with the substrate, the excess of the first precursor is removed, and a second precursor is then applied to react with the substrate surface. The excess of the second precursor is then removed and the process is repeated. In all ALD processes, the substrate is exposed sequentially to a series of reactants that react with the substrate and are kept isolated from each other to avoid CVD or gas phase reactions. An ALD cycle is defined by the steps required to form a single layer of the overall thin-film material; for a process using two precursors a cycle is defined as the first precursor exposure, a purge step, the second precursor exposure, and a second precursor purge step.
A version of ALD processes known as spatial atomic layer deposition (SALD) employs a continuous (as opposed to pulsed) gaseous material distribution from a deposition head. As distributed from the deposition head, the gaseous precursors are separated in space by the flow of an inert gas, rather than being separated in time. While vacuum chambers can be used with SALD, they are no longer necessary due to the physical separation of the gas flows rather than a temporal separation of the precursors within a single chamber. In SALD systems, the required sequential exposures are accomplished by relative movement between the substrate and the delivery head such that any given point on the substrate sees the necessary sequence of gaseous materials. This relative movement can be accomplished by moving a substrate relative to a fixed delivery head, moving a delivery head with respect to a fixed substrate, or moving both the delivery head and the substrate in order to achieve the desired gas exposure at the substrate. Exemplary SALD processes, are described in commonly-assigned U.S. Pat. Nos. 7,413,982, 7,456,429, 7,789,961, and U.S. Patent Application Publication 2009/0130858, the disclosures of which are incorporated herein by reference. SALD enables operation at atmospheric or near-atmospheric pressures and is capable of operating in an unsealed or open-air environment, making it compatible with web coating.
SALD offers considerable promise as a technique for thin film deposition on a range of substrates. However, in spite of its inherent technical capabilities and advantages, a number of technical hurdles still remain. As in all ALD processes, the thickness of the SALD deposited thin-film is controlled by the number of ALD cycles to which the substrate is exposed, where a cycle is defined by the exposure of the substrate to the minimum required reactant and purge gas flows to form the desired thin-film composition. Due to the process being limited to an atomic layer of growth per cycle, repeated cycles are required to deposit a thin-film having an appreciable thickness. In order to effectively achieve repeated cycles, SALD requires either motion of the substrate past the deposition head or the development of complex equipment such that the delivery head moves with its gas connections, relative to the substrate. Thin-films of appreciable thickness can be accomplished by either 1) using a deposition head containing a sufficient number of gas distribution cycles and moving a substrate (or head) in a unidirectional motion relative to the head (or substrate) or 2) using a head with a limited number of cycles and using relative reciprocating motion. In instances where the substrate or the deposition head are moved by a reciprocating movement, there remains a technical challenge to manage the sequence of gas exposures since the substrate can be exposed to the gases in a different sequence during a forward stroke and a backward stroke. Furthermore, in order to deposit a thin-film over an entire substrate, the substrate or the head may have to travel a long distance in order to expose substrate to the process gases. There remains a need to provide alternative arrangements to both the very large deposition heads and long distance motion profiles such that large substrates may be easily coated.
One alternative to a single large deposition head is to use multiple deposition heads, or modules, within a larger deposition section. Commonly-assigned U.S. Pat. No. 8,182,608 (Kerr et al.), which is incorporated herein by reference, relates to an apparatus for maintaining the alignment or positional relationship between at least two modules in an SALD system. U.S. Pat. No. 8,182,608 describes aligning multiple delivery heads in a 1-D array, addressing the ability to coating longer substrates or provide thicker thin-film coatings. While simplifying the manufacturing of the deposition head, it does not address the challenge of making coatings of different thicknesses using the same tool, or the footprint required for providing a large deposition section in a manufacturing environment. Additionally, there remains a need for a way to arrange modular heads to be able to coat wider substrates without coating defects or non-uniformity. Additionally, there remains a need for a motion profile that enables the use of small deposition heads in order to build up a sufficient layer thickness from an SALD. Furthermore, there remains a need for a substrate handling means for coating on roll-to-roll webs that enables exposure of the substrate to multiple SALD cycles during deposition, while simultaneously moving the substrate smoothly from the feed roll to the take-up roll.
In order to function properly, an SALD system must maintain the separation of the reactant gases. Although separated in space and by a purge gas as delivered by the deposition head, the system must be further designed to insure that the gases do not mix in the region between the deposition head and the substrate. Commonly-assigned U.S. Patent Application Publication 2009/0130858 (Levy), relates to an SALD deposition system and method using a delivery head where the distance between the substrate and the deposition head is maintained by gas pressure. In this device, the pressure of flowing reactive and purge gases is used as a means to control the separation between the deposition head and the substrate. Due to the relatively large pressures that can be generated in such a system, gases are forced to travel in well-defined paths and thus eliminate undesired gas intermixing.
The system of U.S. Patent Application Publication 2009/0130858 operates as a gas-bearing SALD system. The gas bearing operation maintains a close proximity of the substrate to the deposition head, and either the substrate or head must be free to move in the direction normal the deposition head. The use of a gas bearing SALD head is advantaged due to the resultant pressure profiles that separate the precursor gasses by the purge gas and prevent undesired gas intermixing. There remains a need for SALD systems that utilize a gas-bearing deposition head to coat large substrates, particularly for depositions systems with small manufacturing footprints. There remains a need to coat long substrates with deposition heads that are considerably smaller than the coating length, both for piece-parts and particularly for roll-to-roll webs; this need further necessitates novel motion control profiles and substrate handling. There remains a further need for roll-to-roll SALD systems that utilize a gas-bearing deposition head having a simple construction, as well as roll-to-roll systems that can manage potential substrate distortions and can isolate the motion needed for deposition from the global motion of the web through the system. Additionally, there remains a need, for a modular system that can accommodate different substrate form factors, including roll-to-roll webs of substrate, and provide a system that is relatively low in cost and easy to use.